A time-division multiplexed (TDM) passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. A TDM-PON typically includes an Optical Line Terminal (OLT) at the service provider's central office connected to a number (typically 32-128) of Optical Network Terminals (ONTs), each of which provides an interface to customer equipment.
In TDM-PON operation, downstream signals are broadcast from the OLT to the ONTs on a shared fibre network. Various techniques, such as encryption, can be used to ensure that each ONT can only receive signals that are addressed to it. Upstream signals are transmitted from each ONT to the OLT, using a multiple access protocol, such as time division multiple access (TDMA), to prevent “collisions”.
A Wavelength Division Multiplexed PON, or WDM-PON, is a type of passive optical network in which multiple optical wavelengths are used to create multiple point-to-point connections and increase the upstream and/or downstream bandwidth available to end users. Instead of an optical power splitter, unpowered optical wavelength multiplexers and de-multiplexers are used. Data encryption is optional, since WDM-PON channels are physically separated by wavelength, and each ONT only receives the data that is intended for it. FIG. 1 is a block diagram illustrating a typical WDM-PON system. As may be seen in FIG. 1, the OLT 4 comprises a plurality of transceivers 6, each of which includes a light source 8 and a detector 10 for sending and receiving optical signals on respective wavelength channels, and an optical combiner/splitter 12 for combining light from/to the light source 8 and detector 10 onto a single optical fibre 14. The light source 8 may be a conventional laser diode such as, for example, a distributed feed-back (DFB) laser, for transmitting data on the desired wavelength using either direct laser modulation, or an external modulator (not shown) as desired. The detector 10 may, for example, be a PIN diode for detecting optical signal received through the network. An optical mux/demux 16 (such as, for example, a Thin-Film Filter—TFF) is used to couple light between each transceiver 6 and an optical fibre trunk 18, which may include one or more passive optical power splitters (not shown).
A passive remote node 20 serving one or more customer sites includes an optical mux/demux 22 for demultiplexing wavelength channels from the optical trunk fibre 18. Each wavelength channel is then routed to an appropriate branch port 24 which supports a respective WDM-PON branch 26 comprising one or more Optical Network Terminals (ONTs) 28 at respective customer premises. Typically, each ONT 28 includes a light source 30, detector 32 and combiner/splitter 34, all of which are typically configured and operate in a manner mirroring that of the corresponding transceiver 6 in the OLT 4.
Typically, the wavelength channels of the WDM-PON are divided into respective channel groups, or bands, each of which is designated for signalling in a given direction. For example, C-band (e.g. 1530-1565 nm) channels may be allocated to uplink signals transmitted from each ONT 28 to the OLT 4, while L-band (e.g. 1565-1625 nm) channels may be allocated to downlink signals from the OLT 4 to the ONT(s) 28 on each branch 26. In such cases, the respective optical combiner/splitters 12,34 in the OLT transceivers 6 and ONTs 28 are commonly provided as passive optical filters well known in the art.
The WDM-PON illustrated in FIG. 1 is known, for example, from “Low Cost WDM PON With Colorless Bidirectional Transceivers”, Shin, D J et al, Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With this arrangement, each branch 26 is allocated a predetermined pair of wavelength channels, comprising an L-band channel for downlink signals transmitted from the OLT 4 to the branch 26, and a C-band channel for uplink signals transmitted from the ONT(s) 28 of the branch 26 to the OLT 4. The MUX/DEMUX 16 in the OLT 4 couples the selected channels of each branch 26 to a respective one of the transceivers 6. Consequently, each transceiver 6 of the ONT is associated with one of the branches 26, and controls uplink and downlink signalling between the ONT 4 and the ONT(s) 28 of that branch 26. Each transceiver 6 and ONT 28 is rendered “colorless”, by using reflective light sources 8, 30, such as reflective semi-conductor optical amplifiers; injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source 8, 30 requires a “seed” light which is used to produce the respective downlink/uplink optical signals. In the system of FIG. 1, the seed light for downlink signals is provided by an L-band broadband light source (BLS) 36 via an L-band optical circulator 38. Similarly, the seed light for uplink signals is provided by a C-band broadband light source (BLS) 40 via a C-band optical circulator 42.
As may be seen in FIGS. 2a and 2b, each of the broadband light sources (BLSs) 36, 40 may be constructed in a variety of different ways. In the BLS of FIG. 2a, a set of narrow-band lasers 44 are used to generate respective narrow band seed lights 46, each of which is tuned to the center wavelength of a respective channel of the WDM-PON. A multiplexer 48 combines the narrow-band seed lights 46 to produce a WDM seed light 50, which is then distributed through the WDM-PON to either the ONTs 26 (in the case of C-band seed light) or the transceivers 6 (in the case of L-Band seed light). If desired, each of the narrow-band lasers 44 may be provided as conventional distributed feedback (DFB) semiconductor laser diodes. Alternatively, multi-channel quantum dot lasers can be used, in which case the number of different laser diodes needed to produce all of the narrow-band seed lights is reduced. Multi-channel quantum dot based lasers are known in the art. In some embodiments, a single multi-channel quantum dot laser may be used to generate all of the desired narrow-band seed lights, in which case the multiplexer 48 is not required.
In the BLS of FIG. 2b, the broadband light source (BLS) is provided by a continuous light source 52 such as a Superluminescent Light Emitting Diode (SLED) that produces a continuous spectrum of light across a wide range of wavelengths. A comb filter 54 generates the desired WDM seed light 50 by filtering the continuous spectrum light emitted by the SLED 52.
In both of the BLSs of FIGS. 2a and 2b, an optical amplifier 58 (for example an Erbium Doped Fiber Amplifier (EDFA)) can be used to amplify the WDM seed light 50. This arrangement is useful for increasing link budget (and thus signal reach), particularly for uplink signals for which the light must traverse the WDM PON twice.
The system of FIGS. 1 and 2 is advantageous in that the light sources 8, 30 are colorless. As a result, a common transceiver configuration can be used for every channel, which facilitates reduced costs via economies of scale and reduced administration However, the requirement for L-band and C-band seed light BLSs and optical circulators tends to increase cost and complexity of the ONT, and so at least partially offsets the benefits of using colorless light sources. In addition, the location of the C-band BLS 40 in the OLT 4 means that light of the uplink signals must traverse the WDM-PON twice, so that the uplink signals received by the transceivers 6 are subject to “round-trip” attenuation. By contrast, light of the downlink signals only traverse the WDM-PON once, and so will inherently require lower BLS power. This implies that the in the performance of the WDM-PON as a whole will be limited by the signal reach of the uplink signals.